The presently disclosed subject matter relates to control of uninterruptible power supplies (UPSs).
An uninterruptible power supply (UPS) is an electrical device that can supply power to a load despite variations in quality and/or availability of utility-based power. One common type of UPS is a double-conversion UPS. A double-conversion UPS receives power from a power source (typically a utility power grid), converts the power from alternating current (AC) to direct current (DC) in a rectifier, and stores at least some of this power in an energy storage device. An inverter changes the DC power from the rectifier and/or the energy storage device into an AC power waveform. The AC power waveform may be supplied to the load.
Because multiple-conversion operation can be relatively inefficient, many UPS devices can operate in an alternative mode to improve efficiency. In one power-saving mode, power to the load may be supplied primarily through a bypass feed path from the utility or other power source to the load. A static switch module (SSM) typically switches the bypass feed path on or off. When several UPSs are connected to the load in parallel, the load sharing during bypass operation may not be actively controlled and thus may be heavily dependent on differences in impedance due to external cable lengths. Under such conditions, the SSMs or other devices of the bypass feed paths may fail open or fail short with no visible effect on the output voltage.